Carbon isotope composition of organic seston and sediments in a Georgia salt marsh estuary

Abstract

The distribution of δ 13 C values for organic seston and sediment was determined in three sounds in the Spartina marsh estuaries along the Georgia coast, which had high, moderate, and low inputs of freshwater. Organic matter in all three sounds had similar carbon isotope compositions, for the most part within the range of marine values ( δ 13 C of −18%. to −24%.). It appears that river flow does not introduce significant quantities of particulate C 3 plant material ( δ 13 C of −25%. to −28%.) to Georgia estuaries. Evaluation of δ 13 C values of estuarine seston and three size fractions of sediment indicated that while Spartina carbon ( δ 13 C of −13%.) can be an important component of organic matter in intertidal sediments (mean δ 13 C of −14.3%. to −20.0%.), it is less so in subtidal sediments (mean δ 13 C of −18.8%. to −21.2%.), and it is hardly present at all in the seston (mean δ 13 C of −24.5%.). δ 13 C values of dissolved inorganic carbon (DIC) in several water samples ranged between −2.5%. and −5.6%., suggesting that the isotope composition of estuarine DIC is influenced by respiratory CO 2 derived from metabolism of 13C-depleted plant carbon. Phytoplankton production utilizing this comparatively light DIC could be a source of relatively negative δ 13 C carbon in the estuary. Additional origins of estuarine organic matter greatly depleted in 13C compared to Spartina carbon remain to be identified.